Conventional mud logging has been used for over 60 years for various purposes, including detection of oil- or gas-bearing sections while drilling. Other information may be obtained by mud logging that can be useful in determining coring and casing points, or for determination of over-balanced or under-balanced drilling conditions. Thus, mud logging is valuable both for economic and safety considerations.
Mud logging services typically provide a continuous reading of hydrocarbons, and use chromatographic analysis to give the concentrations of individual components. One problem with current mud logging systems is that there is a significant amount of error in the measurements, making the results often more qualitative than quantitative.
When a well is drilled, crushed rock and any contained fluids are released and transported to the surface in the drilling fluid. If geologists could separate those formation fluids from the drilling fluids, they could determine the quantity and type of the formation fluids contained in the formation. The accuracy of those determinations has been reduced because of an inability to measure the losses of gases in the rig surface system and the gas extraction mechanism.
The conventional gas logging of wells uses a gas trap, often installed at the possum belly, as the place to install the gas extraction equipment, far from the wellhead. This is the preferred installation spot because is the first one opened and accessible for installing the gas extraction device. The gas composition measured is known to be inaccurate because (i) quantifying the extraction from a classical gas trap has been difficult, and (ii) even if a quantitative extraction device and analyzer is available, the gas losses occurring between the bell nipple and possum belly have previously been unmeasured. Quantitative mud logging systems have been developed that attempt to more accurately identify and measure gas in the recovered drilling fluid, but those systems have been hampered by the unknown amount of gas lost at the rig surface.
In one attempt to gain information about the surface losses, a full-scale 150 bbl test facility was built with flow rates of up to 1000 gallons per minute to be pumped through the bell nipple and down a return line into the possum belly. Metered natural gas was injected into the mud. An ejector module measured gas extracted from open space in the bell nipple and the return line. Additional samples were taken from the possum belly, and compared with the measurements made by the detector module. The study concluded that almost 50% of the gas is lost in the surface system before the drilling fluid reaches the possum belly.
The technique used in the study had significant limitations. Different rig topologies, such as open trough sections, would require different configurations of the measurement equipment. According to the authors, the technique was only usable on water-based drilling fluids. The technique also required two independent analyzers. In addition, the results did not provide good quantitative gas data that resulted in the development of interpretive packages. Such differential techniques imply the installation of a first gas sampling location close to the bell nipple, which is a hard to access location that implies adaptation and/or perforations of the annulus or flow line and involves the cooperation of the drilling contractor for such changes. The modifications required in the area around the bell nipple at the top of the annulus can cause safety and efficiency concerns. In addition, such a location creates maintenance service difficulties.
Techniques such as described above are very laborious and expensive, producing results that may not be applicable on rigs with different topologies. If one attempts to figure out the losses on a pilot rig using the above mentioned technique and then tries to apply a loss formula on further rigs using just the possum belly sampling location, then the results would vary from rig to rig depending on the bell nipple opening to air, the length and inclination of the flow line, different turbulence regimes for the mud flow, etc., making development of a gas losses formula more difficult. Thus, to the inventor's knowledge, the technique described above has never been used in a production environment, but was only intended as a prototype and its use was mostly to point out that such gas losses exist and are quite significant.